The Earth moves in the plane of the ecliptic around the Sun center. Figure A shows the Earth at the moment of the summer solstice. You will notice that the Earth does not sit-up-straight in its own orbit. The polar axis (the line of the North and South Poles) is forever tilted to the plane of the Earth's orbit. This 'tilt' or angle is the whole of the difference between the ecliptic (zodiac) system of coordinates and the equatorial system. If the Earth were not tilted, the two systems would be identical. As it is, there is a difference between longitudes measured along the ecliptic and those longitudes measured along the equator in right ascension.

The diagrams on this page should help you to see the difference between these two systems. Be sure you can understand the following statements:

The Earth always stays in the plane of the ecliptic.

The North Pole of the Earth is tilted toward the plane of the ecliptic by an angle of 23 1/2°.

The polar tilt is permanent, although it changes somewhat over a long period of time.

As the Earth moves around the Sun, the North Pole always points in the same direction.

That direction amounts to the zero-degrees of the zodiac sign Cancer.

The North Pole of the Earth is tilted by a out a 23 1/2° angle toward 0° of Cancer (tropical zodiac).

In fact, the direction of 0° Cancer is defined by the direction toward which the North Pole is tilted (in the Tropical zodiac).

The important idea so far is that the axis of .the Earth is frozen or fixed in space, no matter where the Earth happens to be in its orbit around the Sun. Here are some other facts to consider in relation to these same diagrams:

The seasons result from the 'tilt' of the North Pole into or toward the Sun.

At the summer solstice, the North Pole is tilted most toward and most aligned with a vertical light ray coming from the Sun.

The polar axis of the Earth is in line with a vertical light ray only twice a year, at the solstices.

At the equinoctial points (Spring and Fall), the polar axis of the Earth is at right-angles or 'square' to a vertical light ray coming from the Sun.

At all other times of the year besides these four cardinal points, the angle between the polar axis of the Earth and a vertical light ray coming from the Sun will be somewhere between 0° and 90°.

A parallel of latitude on the Earth at 23 1/2° North is called the Tropic of Cancer since this line marks the "high-water" point for the summer solstice, after which the Sun declines in strength.

A similar point at 23 1/2° South latitude is called the Tropic of Capricorn.

The arctic and anarctic circles are those circles near the Earth's poles defined by the difference between the North Pole of the Earth and the north ecliptic pole.

Half of the Earth is always in darkness.

The ecliptic and equatorial systems are both measured from the same point -- zero-degrees Aries or the Vernal Equinox. Longitude is measured from this point along both the celestial equator and the ecliptic in degrees from 0° to 360°. The Vernal Equinox or 0° Aries point originates or is defined as the intersection of the ecliptic and the celestial equator and this intersection (of these two infinite planes) creates an axis and a pair of nodes set in space.

It must be kept in mind that it does not matter that the orbital plane of the Earth is very large and the physical equator of the Earth relatively small. In spherical coordinate systems, we are not concerned with the size of the structures, but with the angles and planes of orientation of these structures. We extend the equator of the Earth out until it reaches the heavens -- infinity. We extend the orbital plane of the Earth out until it reaches the heavens -- infinity. The points and axis where these two planes intersect in the heavens is all that we are concerned about.